Fatal Uncaught Error: Class “DOMDocument” not found

I just moved a WordPress site from shared hosting to a VPS. In the process we upgraded to PHP 8.1. When I try to load our home page, it crashes, and I get this in the apache error logs:

Got error 'PHP message: PHP Fatal error:  Uncaught Error: Class "DOMDocument" not found in wproot/wp-content/plugins/editorplus/includes/class-editorplus-styles-manager.php:61

Stack trace:
#0 wproot/wp-includes/class-wp-hook.php(309): EditorPlus_Styles_Manager->prepare_blocks_for_styles()
#1 wproot/wp-includes/plugin.php(191): WP_Hook->apply_filters()
#2 wproot/wp-includes/class-wp-block.php(284): apply_filters()
#3 wproot/wp-includes/blocks.php(901): WP_Block->render()
#4 wproot/wp-includes/blocks.php(939): render_block()
#5 wproot/wp-includes/class-wp-hook.php(307): do_blocks()
#6 wproot/wp-includes/plugin.php(191): WP_Hook->apply_filters()
#7 wproot/wp-includes/post-template.php(253): apply_filters()
#8 /...'

If I disable the plugin, everything seems to work fine. But, of course, we want the plugin! 🙂

In theory this could be a permissions problem or something, but I feel like I have that all set correctly. Any guesses? Googling the error message turned up nothing, so this seems to be either something very new or something I’ve got configured wrong. I’d appreciate any hints you can offer.